1. Field of the Invention
The present invention relates to an electric rotating machine.
2. Description of the Prior Art
In the conventionally electric rotating machines having a field rotor in which a field winding is wound round on a pole core, it has been a known technique that a permanent magnet is mounted between claw-shaped magnetic poles adjacent to each other in circumferential direction of the pole core (as disclosed in, for example, the Specification of Japanese Patent No. 2548882).
The permanent magnet not only reduces leakage flux between the claw-shaped magnetic poles but also increases effective magnetic flux that contributes to power generation by interlinkage of the magnetic flux, thus improving power output of the electric rotating machine. The rotor is arranged to have a ventilation space for separating the permanent magnet and the field winding from each other with a predetermined radial distance, and an air-cooling system introducing outside air is adopted for cooling the rotor. Accordingly, the permanent magnet is necessarily exposed to outside air. For the purpose of protection, it has been popular in the conventional electric rotating machines to apply any coating such as resin coating to the permanent magnet.
In the conventional electric rotating machines of mentioned above arrangement, a problem exists in that, in the process of assembling the rotor, the coating comes off at the time of mounting the permanent magnet or due to mixture of foreign matter from outside or heat deterioration, and the permanent magnet rusts eventually resulting in deterioration of characteristics thereof.
In the conventional electric rotating machines mounted with a permanent magnet, because of its arrangement, the permanent magnet may inhibits airflow inside the rotor, making the cooling efficiency down, and furthermore any reversible or irreversible demagnetization may occur due to temperature rise caused by heat generation of the permanent magnet.
Moreover, because of poor heat conduction of the resin coating, in the conventional electric rotating machines, a further problem exists in that cooling efficiency for the permanent magnet is poor, and temperature of the permanent magnet rises due to heat received from the stator coil and eddy current generated in the permanent magnet, occurring the reversible demagnetization, reducing amount of magnetic flux resulting in deterioration of characteristics, which further brings about reduction in magnetic flux density of the permanent magnet due to opposing magnetic field eventually resulting in irreversible demagnetization.